Improving the fuel efficiency of vehicles has recently become an urgent necessity to control CO2 emissions from the viewpoint of global environmental conservation. Thus, weight saving by reducing the thickness of parts has been required. In addition, improvements in safety, particularly crashworthiness of car bodies, have been required to ensure the safety of occupants in the event of a crash. Thus, both weight saving and strengthening of car bodies have been actively pursued.
For a car body to satisfy light weight and high strength at the same time, it is said to be effective to increase the strength of the material for automobile parts and to reduce the sheet thickness to such an extent that stiffness does not become a problem, thereby saving weight. High strength steel sheets have been recently actively used for automobile parts. In particular, there have been increasing demands for high strength steel sheets with a tensile strength of not less than 590 MPa. Weight saving effects are higher as the steel sheet used has higher strength. However, the higher the strength of a steel sheet, the more likely the occurrence of problems such as deteriorated shape fixability, an overload to a mold and the occurrence of cracks, necking or wrinkles being encountered during forming of the steel sheet.
To remedy these problems, according to known techniques, the amounts of elements incorporated into the steel and form a solid solution are controlled, and a strain aging hardening phenomenon that occurs during a baking step at 170° C. for 20 minutes is utilized such that the steel is worked and formed while its strength is low and its ductility is high and, after being formed, the steel is increased in strength through the baking step.
Japanese Unexamined Patent Application Publication No. 2005-206943 discloses a high strength hot-rolled steel sheet which contains C at 0.01 to 0.12%, Mn at 0.01 to 3% and N at 0.003 to 0.020%, has a bainite single phase or a mixed microstructure of a bainite phase and a second phase, and contains a controlled amount of solute nitrogen, thereby achieving excellent bake hardenability and aging resistance at ambient temperature.
Japanese Unexamined Patent Application Publication Nos. 2009-41104 and 2003-49242 disclose steel sheets with excellent strain aging hardenability and ductility which contain a controlled amount of solute nitrogen and have a microstructure including a ferrite phase at an area ratio of not less than 50%.
Japanese Unexamined Patent Application Publication No. 2004-76114 discloses that a high strength hot-rolled steel sheet with excellent bake hardenability is obtained by configuring the steel sheet to contain at least 3% of retained austenite.
Of the steel sheets described in JP '943, those which are free from such elements as chromium, molybdenum and nickel are insufficient in strength with a strength value being less than 590 MPa. Steel sheets to which such elements as chromium, molybdenum and nickel have been added exhibit a strength of not less than 590 MPa, but are insufficient in terms of costs and recyclability because of the addition of such elements. The higher the strength of a steel sheet, the smaller the increase in deformation stress (BH value) before and after an aging treatment, the difference in tensile strength (TS) (BHT value) before and after the aging treatment, and the hole expanding ratio (λ). However, that publication does not consider bake hardenability or stretch-flangeability at a steel sheet strength of not less than 590 MPa.
The steel sheets described in JP '104 and JP '242 are poor in stretch-flangeability because their microstructures are multiple phase microstructures mainly formed of a soft ferrite phase and a hard phase such as a martensite phase. In the steel sheet described in JP '114, good stretch-flangeability cannot be obtained because the steel sheet contains retained austenite which is very hard.
In view of the above circumstances, it could be helpful to provide a high strength hot-rolled steel sheet with a tensile strength of not less than 590 MPa which exhibits excellent bake hardenability and stretch-flangeability, and to provide a method for manufacturing such steel sheets.